1. Field of the Invention
The present invention relates to a power supply protection system, and more particularly to a power supply protection system applied to an optical subscriber network having subscriber transmission nodes.
In most cases, a central office is located at the center of each big city. In this case, it is not economical for the carrier to distribute transmission lines separately to reach each of the subscriber if they live in the suburbs of the big city.
To cope with this, it is a recent trend to mount, for the subscribers living in the suburbs, a small capacity subscriber accommodation apparatus for suburban use, i.e., a remote office, where the remote office and the central office are connected through a single optical fiber transmission line.
In the above case, it is further proposed to connect the remote office and the subscribers in the suburbs with the use of an optical fiber transmission line, whereby an optical subscriber network is set up. This is referred to in North America, as FITL which means Fiber In The Loop, and can cope with a future broadband ISDN.
The present invention refers to a power supply which is to be adopted to the above mentioned optical subscriber network.
2. Description of the Related Art
In a conventional subscriber network, transmission signals are transferred on metallic wires, and therefore it is possible to supply power to the subscribers by commonly using the metallic wires. However, in the optical subscriber network no such metallic wires are used, and accordingly, power must be supplied independently to each subscriber separately from the optical fibers.
Under the above circumstances, it has been proposed to supply commercial power to each subscriber's terminal equipment independently. This measure, however, is not suitable, because, if the commercial power supply is cut off, normal communication for the related subscriber can no longer be maintained.
This being so, it has been proposed to supply power to each subscriber from a remote power supply apparatus mounted in the remote office. In this case, it is possible to distribute many pairs of power supply lines to each subscriber respectively. However, the power supply lines play a role merely to carry a power and no role in transferring information as does the optical fiber transmission line. From this viewpoint, it is preferable to distribute a single pair of power supply lines commonly used by a plurality of subscribers. This enables a considerable reduction in the cost of constructing power supply facilities.
As will be explained in detail hereinafter, each subscriber transmission node is located along the side of a road and a pair of power supply lines is buried at a shallow level along the road. Therefore, the power supply lines and the subscriber transmission nodes are in an environment liable to be destroyed by a road construction, wheels of a car and the like. Thus, there is a high possibility for the power supply lines and the subscriber transmission node to suffer from trouble in the power supply such as a ground fault, a short-circuit fault, an open-circuit fault and so on.
In general, it is impossible to perfectly prevent such trouble in the power supply from occurring, and therefore, if such trouble does occur, the overall subscribers suffer from the trouble in common, and thereby, a service interruption may occur for all the subscribers at one time. This is a considerable problem.